Some known wind turbines include machines for converting variable speed mechanical input, from blades of the wind turbine, into electric power that is compliant with an electrical grid. For example, the wind turbines may include a DFIG wind generator for converting the variable speed mechanical input.
Some conventional DFIG wind generator rotors have a floating neutral point. This floating neutral point is frequently provided by a device known in the art as a wye ring. The wye ring is typically made from a copper bar and is located at the non-drive end (NDE) of the generator.
A typical DFIG wind generator is constructed with coils in the rotor which are electrically connected by the wye ring creating the rotor circuit, allowing the generator to operate normally. The rotor coils and connections are a rigid piece due to the vacuum process impregnation (VPI) required to cure the coils. Wye ring is also often a source of premature failure in DFIG wind generators. The VPI process, however, makes it virtually impossible to remove existing wye ring connections for repairs.
Most wye ring premature failures are related to lack of possibility and space to accommodate thermal expansions caused by frequent speed variations of the generator since wye ring and coils connection becomes a rigid piece due to associated manufacturing processes. By way of background, DFIG wind generators are usually designed to operate at different speeds, based on existing wind conditions. The varying speeds of the turbine therefore the generator create thermal expansions producing different levels of stress specifically concentrated on the generator's rotating parts, such as the rotor.
The stresses on the rotor, along with the rotor's wye ring inability to accommodate thermal and dynamic expansions, fatigue brazed connections between the wye ring and its rotor connection points. As a result, breaks or cracks develop, creating discontinuities in the wye ring and rotor connection points.
With the occurrence of one crack, the generator can continue to function satisfactorily since current can still reach all three (i.e., associated with each phase) rotor connection points. That is, with only one crack, the existing wye ring will continue closing the rotor's electrical connections. The continued closing of the electrical connections can minimize the impact of the crack to the generator's performance.
However, if a second crack occurs in the wye ring, which could occur within hours, weeks, or years of the first crack, at least one segment (e.g., one phase) of the rotor windings will be disconnected from the floating neutral point. This disconnection will result in catastrophic failure of the generator, immediately disrupting all power generation. In most cases, the rotor coils are actually burned as a result of this second crack.
These failures can be particularly problematic and difficult to remedy. In the case of windfarms, for example, a customer may deploy thousands of DFIG wind generators. Currently, there are few practical and effective solutions to address rotor wye ring failures in the field, especially when considering the large numbers of generators deployed in wind farms.
Conventional solutions, especially with the inherent difficulties associated with the VPI process, usually involve prolonged downtime, and the risk of damaging insulation of the coils and functional connections. Certainly, there are no preventive maintenance solutions that can be applied in the field with the generator mounted in the tower.